As is known in the art, surveillance radars within the United States National Airspace System (NAS) detect the location of aircraft targets and report that information to En Route and Terminal Automation systems for aircraft tracking and subsequent display to air traffic controllers. Radar target report messages include the range and azimuth of the aircraft target relative to the location of the radar. Beacon radars provide additional information including aircraft-reported altitude and aircraft-reported transponder code. However, the absolute time of radar measurement is not included in the radar target reports transmitted by existing NAS radars to the automation systems. The lack of radar measurement time information with the target report messages introduces errors in the estimate of actual aircraft location being tracked by automation systems for the benefit of the air traffic controller. While some conventional automation systems estimate the radar time of measurement using known techniques, significant errors remain in the estimation of aircraft position. Aircraft separation must be large enough to account for this estimation error.
The existing NAS includes many radar types, including ARSR-3, ARSR-4, ASR-8, ASR-9, ASR-11, GPN-66, etc. Modifying each of these radar types to include the time of radar measurement would be a difficult, costly and impractical task. For example, each radar type would need to be interfaced to a real time clock source. The radar interface protocol would need to be changed to accommodate the addition of a field to contain the radar time of measurement. The radar internal processing would need to be updated to process the newly-interfaced real time information and to format the radar targets in the revised protocol. In addition to the radar modifications, en route and terminal automation systems and other users would need to be modified to accommodate the revised protocol. Taking into consideration that existing radars typically provide data to a variety of users, the radar would need to support both the existing protocol and the new protocol simultaneously until all users have adopted the new protocol. While protocols that include radar measurement time, for example ASTERIX (All Purpose STructured Eurocontrol Radar Information Exchange) are beginning to be adopted internationally, they have not been used in the NAS for the above-mentioned reasons.